Science · Class 7
Physical and Chemical Changes
Rusting, crystallisation, and distinguishing physical vs chemical change.
4 sections15–25 min read6 MCQs · 2 examples
Chapter Overview
Physical changes alter form or state without new substance. Chemical changes form new substances with different properties. This chapter covers rusting, crystallisation, and protecting iron from corrosion.
Types of Changes
- Physical: melting ice, tearing paper, dissolving sugar — reversible often
- Chemical: burning, rusting, curdling — new substance, usually irreversible
- Signs of chemical change: colour, gas, heat/light, precipitate
Rusting
- Iron + oxygen + water → rust (hydrated iron oxide)
- Prevention: painting, greasing, galvanisation, alloying (stainless steel)
- Ships and bridges need anti-rust coating
Crystallisation
- Impure solid dissolved in minimum hot solvent
- Cool slowly — pure crystals form
- Example: copper sulphate crystals from solution
Solved Examples
Step-by-step solutions — read each step before checking the final answer.
Classify changes
Classify: melting wax, burning coal, dissolving salt.
- 1Melting wax — physical (same substance)
- 2Burning coal — chemical (CO₂ formed)
- 3Dissolving salt — physical
Answer
Physical: wax melt, salt dissolve | Chemical: burning coal
Prevent rust
How does galvanisation protect iron?
- 1Zinc coating on iron
- 2Zinc corrodes preferentially (sacrificial)
- 3Barrier from air and moisture
Answer
Zinc layer prevents rusting
Key Points to Remember
- ✓ Photosynthesis is chemical change
- ✓ Cutting wood is physical
- ✓ Rusting needs both air and water
Exam Tips
- • Give two examples each of physical and chemical changes
- • List rust prevention methods
- • Describe copper sulphate crystallisation
Practice MCQs — Physical and Chemical Changes
Test your understanding with topic-wise multiple choice questions. Explanations appear after each answer.
Question 1 of 6Score: 0/0
Rusting is a: